The present invention relates to an image data processing device in which an image data is compressed and stored in a memory.
As an image data processing device, there has been known an electronic still camera in which a photographed still image is converted to a digital image signal in a two-dimensional photoelectric conversion element, the digital image signal being then processed and recorded in a memory. The recorded still image is reproduced on a TV screen.
Such electronic still cameras have been attracting public attention for a reason that these cameras change a photographed image into a digital signal which can be applied for wider variety of devices than conventional film camera, consequently promising future developments.
As a memory for storing a digital image signal, there have been proposed a magnetic sheet in the form of a disk and a memory card carrying a solid-state memory. However, a magnetic sheet has not been considered to be preferable to actually use as a memory for an electronic still camera due to the fact that to record image signals on a magnetic sheet, the position of recording head and the rotation of magnetic disk are required to control with very high precision, which consequently makes it impossible to give an electronic still camera high portability and durability and with low cost which are usually needed for cameras.
It has been seen that a memory card carrying a solid-state memory has advantages of requiring a smaller number of mechanical parts to record image signals, and of reducing the size and weight. However, a solid-state memory carried in a memory card has a small storage capacity. Accordingly, greater number of solid-state memories are required to store greater number of image signals, which results in higher cost.
To overcome this drawback, signal compression has been proposed in which an image signal is compressed and stored in a memory, and expanded when reproducing. This signal compression makes it possible to store a greater number of image signals in a limited storage capacity.
In view of storage capacities of presently available solid-state memories, however, it is necessary to compress a image signal at a considerably high ratio. For example, to store a digital signal produced by digitizing one field of image signal at a sampling frequency three times as much as chrominance subcarrier frequency, a storage capacity of about 1.5 mega bits is required. Accordingly, in the standard case of storing digital signals for 50 fields in one memory medium, the memory medium is required to have a capacity of 75 mega bits.
In signal compression, there are two ways. One is a reversible compression which involves no reduction of the amount of information. The other is an irreversible compression which involves reduction of the amount of information. The reversible compression has a maximum compression ration of 1/3, and accordingly requires a memory having a capacity of 25 mega bits to store 50 fields. If used static RAMs (hereinafter called SRAM), it is necessary to use 25 SRAMs to store 50 fields. This is undesirable in aspects of size and cost. On the other hand, the irreversible compression has a maximum compression ration of 1/10. Accordingly, it is sufficient to use 8 SRAMs to store 50 fields. This is desirable in commercial aspect. The irreversible compression is accomplished by using differential pulse-code modulation (DPCM), or discrete cosign transformation (DCT), for example.
However, when an image signal is compressed at a high compression ratio, the quality of the image deteriorates. In other words, a processed image becomes considerably different from the original photographed image. When signal compression is carried out using the above-mentioned DCT in which less number of bits are allocated to high frequency components of the image signal than to the other frequency components, most high frequency components of the image signal are lost through the compression-expansion process, making a reproduced image as a whole blur.
Japanese Unexamined Patent Publication No. 63-286078, for example, discloses an electronic still camera having two recording modes. One is a compression mode in which an image signal is recorded after being compressed. The other is a non-compression mode in which an image signal is recorded without being compressed. The photographer can select one of the two modes to obtain image quality and storage capacity suitable for his photographic purpose. In this camera, also, the selected mode for each photography is recorded on a memory, and reproduction is performed in accordance with the recorded mode selection. This camera employs a signal compression in which an image signal is compressed by thinning picture element signals. Accordingly, all the portions of one picture are reproduced at the same definition, or the same compression ratio. Therefore, this camera have the above-mentioned problem that the reproduced image is out of focus as a whole.
Japanese Unexamined Patent Publication No. 63-299680 discloses a method of digitizing a photographed image signal in which a background portion and a non-background portion of the subject view are discriminated based on the difference between the highest luminance and the lowest luminance in each portion, and image signals corresponding to their respective portions are digitized at different bits. However, it should be noted that in this method, compression ratio is not controlled in accordance with focussing condition. Also, it could be seen that this method needs a complicated circuit for discriminating a background portion and a non-background portions.
Also, there have been proposed devices for erasing image data stored in a solid-state memory. For example, Japanese Unexamined Patent Publication No. 1-185078 discloses an electronic still camera in which a desired image data in a memory card can be erased in accordance with address, and the remaining image data are succeedingly transferred toward the portion which has been emptied by the erasure so that a new emptied portion is produced in a last portion of the memory area.
Japanese Unexamined Patent Publication No. 1-243686 discloses a method for erasing an image data. This erasure method is employed for an electronic still camera carrying a memory card including SRAMs. In this erasure method, when a new image data is stored in the memory card, the new image data is stored over an appropriate old image data already record on the memory card by rewriting administrative data for the old image data. In other words, the old image data is not erased before recording the new image data. Specifically, the memory includes a great number of memory bits which are divided into a number of blocks every a predetermined number of memory bits. An image data is stored by using a plurality of blocks. Administrative data, such as the number of blocks which are used to store each image data, and the serial number or name of the used blocks for each image data, is recorded in a specified memory area. When a particular image data to be erased is instructed, only the administrative data for the particular image data is erased, the particular image data itself is erased when a new image data is written over the particular image data.
Japanese Unexamined Patent Publication No. 1-165267 discloses an electronic still camera in which a photographed image data is recorded in a volatile memory, such as RAM, and displayed in the viewfinder of the camera to enable the photographer to confirm the photographed image before recording it in a memory card.
As mentioned earlier, a solid-state memory provided in a memory card does not have a large storage capacity. For this reason, digitized image data is compressed and stored to utilize a limited storage capacity of the memory card at its maximum.
In recent years, there have been produced memory chips having relatively large storage capacity. Also, it has been proposed a memory card comprising a flash EEPROM (e.g., shown in a Japanese Magazine "Nikkei Electronics" Apr. 4, 1988) which does not require a back-up power supply in order to store image data, instead of the aforementioned SRAM and the like.
However, although a flash EEPROM has an increased storage capacity of the memory card, it is known that data stored in a EEPROM cannot be erased but in the lump or in several kilo bits. There could be seen to be a possibility that the capacity of a memory area holding an image data does not correspond to the capacity of an erasing unit of blocks. Accordingly, it is very difficult to erase one of adjoining two image data a front portion of one and a rear portion of the other of which are recorded in a common block. On the other hand, in the case of recording image data in a specified erasing unit of blocks regardless of the amount of image data to facilitate erasing, the storage capacity cannot be utilized at its maximum.
For such memory means as the aforementioned EEPROM in which image data cannot be erased but in a unit of blocks, accordingly, there has been desired data erasure means which enable recorded image data to be erased in a unit of one picture with assuring the utilization of storage capacity at its maximum.
The data erasure means of the above-mentioned Japanese Unexamined Patent Publication No.1-185078 is useful for a memory in which a desired image data stored in the memory can be erased by designating the address of the desired image data. However, this erasure means is not useful for a memory in which image data cannot be erased but in a unit of blocks, and cannot provide the desired erasure means of enabling image data to be erased in a unit of one picture with assuring the utilization of storage capacity at its maximum.
Also, in the image data erasure means of the above-mentioned Japanese Unexamined Patent Publication No. 1-243686, when erasing a specified image data, only administrative data for the specified image data is erased, but the specified image data itself is not erased. Accordingly, this data erasure means is impossible to employ for a flash EEPROM in which an old image data is necessary to erase before recording a new image data. Therefore, this data erasure means cannot provide the desired erasure means of enabling image data to be erased in a unit of one picture with assuring the utilization of storage capacity at its maximum.
The electronic still camera disclosed in the aforementioned Japanese Unexamined Patent Publication No. 1-165267 enables an photographer to monitor a photographed image in the viewfinder and record only the image he wants to record in the memory card. However, this camera is not provided with data erasure means for utilizing the storage capacity at its maximum by erasing already stored image data.
Accordingly, it is an object of the present invention to provide an image data processing device which has overcome the above-mentioned drawbacks.
It is another object of the present invention to provide an image data processing device which makes it possible to store image data at a maximized utilization of storage capacity of a memory without involving a reduction of the quality of image.
Also, it is another object of the present invention to provide an image data processing device which makes it possible to erase image data stored in different compression ratios every one picture.